Harnessing Innate Immunity for Vaccination
A hallmark of the immune system is its ability to launch qualitatively different types of immune responses. Thus for example, T-helper 1 (or Th1) immune responses stimulate cytotoxic “killer” T cells, which kill virally infected cells or tumors. In contrast, T-helper 2 (or Th2) responses are associated with antibody production, particularly secretion of IgE antibodies, which confer protection against extracellular parasites or bacteria or toxins. Furthermore, T regulatory responses can suppress over exuberant immune responses, and thus limit the immune pathology caused by allergies, autoimmunity, transplant rejection, or sepsis like symptoms. Given the existence of such diverse types of immune responses, and their differential roles in conferring effective protection against viruses, tumors, extracellular parasites and bacteria, and in regulating deleterious immune responses in allergies, autoimmunity, transplantation and sepsis, a “rosetta stone” of modern immunology is to learn how to induce optimally effective immune responses in various clinical settings.
In this context, recent advances in immunology have revealed a fundamental role for the innate immune system in controlling both the quality and quantity of immune responses (Pulendran & Ahmed, Cell, 2006, 124:849-863). Thus, it has long been known that the immune system is unresponsive to most foreign proteins that are injected in a soluble, deaggregated form, but when injected together with immune-stimulating substances called “adjuvants,” these foreign proteins can induce robust immunity. In fact it was known that the nature of the adjuvant is what determines the particular type of immune response that follows, which may be biased towards cytotoxic T-cell responses, antibody responses, or particular classes of T-helper responses (Pulendran, Immunol. Rev., 2004, 199:227-250; Pulendran, J. Immunol., 2005, 175:2457-2465; Pulendran & Ahmed, Cell, 2006, 124:849-863). Despite the importance of adjuvants, there is only one adjuvant, alum, licensed for clinical use in the United States, and most other experimental adjuvants consist of crude extracts of microbes or bacteria, which induce potent activation of immune cells, but also result in toxicities. Until recently, the mechanism of action of such adjuvants was not understood. However, recent advances in innate immunity have offered a conceptual framework with which to understand how adjuvants function. Central to this issue is a rare but widely distributed network of cells known as dendritic cells (DCs), which constitute an integral component of the innate immune system. DCs, which have been called ‘Nature's adjuvants,’ express receptors which can recognize components of microbes and viruses. Such receptors include the Toll-like receptors (TLRs), C-type lectins, and CATTERPILLAR proteins, which can “sense” microbial stimuli, and activate DCs and other immune cells (Pulendran, Immunol. Rev., 2004, 199:227-250; Pulendran, J. Immunol., 2005, 175:2457-2465; Pulendran & Ahmed, Cell, 2006, 124:849-863]. It is now clear that DCs play essential roles in orchestrating the quality and quantity of the immune response.
There are currently some 13 TLRs described in mammals. Activating distinct TLRs on DCs induces qualitatively different types of immune responses (Pulendran, et al, 2001, supra; Dillon et al, 2004, supra; Agrawal et al, J. Immunol., 2003, 171:4984-4989; Dillon et al, J. Clin. Immunol., 2006, 116:916-928). Thus, activating most TLRs can induce Th1 responses; activating TLR3, 7 or 9 can induce cytotoxic T cells that kill virally infected cells and tumors; and emerging evidence suggests that activating TLR2 induces Th2 responses, (which are associated with antibody responses that offer protection against viruses or extracellular bacteria or parasites), or even T regulatory or tolerogenic responses, (which suppress over exuberant immune responses, and thus offer protection against unbridled immunity in allergies, autoimmunity, sepsis, and transplantation). As such, DCs and TLRs and other recognition receptors, represent attractive immune modulatory targets for vaccinologists and drug developers. Thus learning how to exploit fundamental elements of the innate immune system such as DCs and TLRs, is of paramount importance in the development of novel drugs and vaccines.
An important corollary to this notion is that the vast majority of vaccines which have been developed over the past 200 years, (since the first recorded vaccination trial of Edward Jenner), have been developed empirically. Therefore, despite their successes in controlling various scourges such as smallpox, polio, TB and yellow fever, we have no knowledge of the scientific rationale for how these vaccines stimulate such effective immunity. For example, the yellow fever vaccine 17D [YF-17D] is one of the most effective vaccines known. Since its development more than 65 years ago, it has been administered to over 400 million people globally. Despite its success, the mechanism of its action is not known. Therefore, as stated above, the spectacular advances in innate immunity which have occurred in the last six years or so, offer us a new vision with which to understand the modus operandi of such “gold standard” vaccines, with a view to using such knowledge to devising future vaccines against emerging and re-emerging infections of the 21st century. In this context, our recent findings suggest that the highly effective Yellow Fever Vaccine (YF-17D) is a potent stimulator of DCs, and multiple TLRs, including TLR 2, 7, 8 and 9 (Querec et al., J. Exp. Med., 2006, 203:413-421). Given, the different types of immune responses triggered by the distinct TLRs (Pulendran et al., 2001, supra; Agrawal et al., J. Immunol., 2003, 171:4984-4989; Dillon et al, 2004, supra; Dillon et al, J. Clin. Immunol., 2006, 116:916-928), it was tempting to speculate that by activating multiple TLRs, YF-17D was inducing a broad spectrum of immune responses. Indeed, our data suggests that YF-17D triggers a broad spectrum of innate and adaptive immune responses (Th1, Th2, cytotoxic T cells, neutralizing antibody), and that distinct TLRs control different types of this polyvalent immunity (Querec et al., J. Exp. Med., 2006, 203:413-421). Eliciting such a broad spectrum of immune responses is also likely to be beneficial in designing vaccines against other infections, against which no effective vaccines currently exist, such as HIV, HCV, malaria, TB, influenza, anthrax and Ebola, or against tumors. Thus, strategies for designing future vaccines against emerging or re-emerging infections might benefit from incorporating multiple TLR ligands plus antigens, plus immune modulatory agents, in order to induce multi-pronged immune responses. Therefore, an important challenge is the development of delivery systems which are capable of delivering such immune modulatory agents in vivo.
As described herein, polyketal (PK) particles are a new class of biomaterials that hydrolyze in a controllable manner at physiological pH values and degrade into neutral compounds.
Delivery Systems for Novel Vaccines
Drug delivery vehicles based on polyesters and polyanhydrides have been widely used for the sustained release of therapeutics because of their excellent biocompatibility profiles and slow hydrolysis rates (Anderson, J. M. et al., Adv. Drug Delivery Rev., 1997, 28:5-24; Jain, R. A., Biomaterials, 2000, 21:2475-2490; Mathiowitz, E. et al., J. Appl. Polym. Sci., 1988, 35:755-774; Berkland, C. et al., J Controlled Release, 2004, 94:129-141). However, numerous medical applications, such as targeting the acidic environment of lysosomes and tumors, require drug delivery systems that undergo rapid, pH-sensitive degradation (Stubbs, M. et al., Mol. Med. Today, 2000, 6:15-19; Leroux, J.-C., Adv. Drug Delivery Rev., 2004, 56:925-926). The majority of degradable polymers used for drug delivery cannot fulfill this requirement because they are composed of ester linkages, which degrade by base-catalyzed hydrolysis at physiological pH values. Particles made of ester based materials, such as Poly(lactic-glycolic acid) (PLGA), polyorthoesters, and polyanhydrides, all generate high quantities of acid when they degrade. This causes degradation of protein and DNA therapeutics and the degradation also takes weeks to months. Because the life span of mature DCs is around 2 days these materials are not ideal for vaccine development. Recently, pH sensitive hydrophobic microparticles based on poly(orthoesters) and poly(beta-amino esters) have been successfully used for intracellular drug delivery and tumor targeting, thus demonstrating the potential of acid-sensitive biomaterials for drug delivery (Heller, J. et al., Biomacromolecules, 2004, 5:1625-1632; Heller, J. et al., Adv. Drug Delivery Rev., 2002, 54:1015-1039; Berry, D. et al., Chem. Biol., 2004, 11:487-498; Potineni, A. et al., J Controlled Release, 2003, 86:223-234). Consequently, there is great interest in developing new strategies for the synthesis of pH-sensitive biodegradable polymers.
Vaccines based on recombinant proteins, peptide antigens, or DNA vaccines encoding such vaccine antigens, have tremendous therapeutic potential against infectious diseases and tumors, in which the antigenic epitopes have been defined. Such vaccines have been capable of generating protective immunity against infectious diseases, in animal models, and numerous clinical trials with such vaccines are currently in progress (van Endert, P M, Biologicals, 2001, 29:285-8; Purcell, A W et al., Journal of Peptide Science, 2003, 9:255-81; Shirai, M. et al., Journal of Virology, 1994, 68:3334-42; Hunziker, I P et al., International Immunology, 2002, 14:615-26). However, despite their promise, a major challenge concerns the efficient delivery of peptides, proteins, DNA vaccines and adjuvants, so as to target the appropriate type of antigen presenting cell in order to launch an effective immune response. Although promising results have been obtained with peptide vaccines composed of lipid conjugates and PLGA microparticles, there is still a great need for the development of new peptide vaccine delivery vehicles (Ertl, H C J et al., Vaccine, 1996, 14:879-85; Jackson, D C et al., Vaccine, 1997, 15:1697-705).